1. Technical Field
The embodiments described herein relate to cryptographic communication and in particular to systems and methods for enhancing the security of cryptographic communications, while still allowing the use of common cryptographic schemes.
2. Related Art
Symmetric cryptography is often used to protect information transmitted over a network or via radio or light transmissions. The protections afforded by symmetric cryptographic can include confidentiality, integrity, source authentication, and replay protection. In some applications, it may be necessary to also protect the privacy of the user of the transmitting device.
Radio frequency Identification (RFID) systems are an example of a type of system in which symmetric cryptography can be used to provide secure communications. For example, symmetric cryptography can be used to protect the confidentiality of a unique identification number transmitted between an RFID tag on an identification card, document, etc., and an RFID reader. In such instances, each RFID tag can have a unique secret key, used as input to the symmetric cryptographic algorithm that is used to encrypt data such as a unique identifier on the tag prior to transmission.
In some cases, the cryptographic parameters used for the encryption can be transmitted with the cipher text to enable decryption at the receiver. Such parameters can include an Initialization Vector (IV) used with some symmetric encryption modes. These cryptographic parameters can contain information that could compromise the identity of the holder of the identification card or document if, e.g., they were intercepted during transmission. For example, if a counter is used as IV for a symmetric algorithm, e.g., in counter mode or cipher feedback mode, then the current value of the counter used as the IV is a form of identifier. If this identifier were to be intercepted, then it is possible that the cryptography can be defeated and the user's identity, or other sensitive information, could be compromised.
By way of background, FIG. 1 is a diagram illustrating an exemplary RFID system 10. In system 10, RFID interrogator 102 communicates with one or more RFID tags 110. Data can be exchanged between interrogator 102 and RFID tag 110 via radio transmit signal 108 and radio receive signal 112. RFID interrogator 102 comprises RF transceiver 104, which contains transmitter and receiver electronics, and antenna 106, which are configured to generate and receive radio transit signal 108 and radio receive signal 112, respectively. Exchange of data can be accomplished via electromagnetic or radiative coupling in the RF spectrum in combination with various modulation and encoding schemes.
RFID tag 110 is a transponder that can be attached to an object of interest and act as an information storage mechanism. In many applications, the use of passive RFID tags is desirable, because they have a virtually unlimited operational lifetime and can be smaller, lighter, and cheaper than active RFID tags that contain an internal power source, e.g. battery. Passive RFID tags power themselves by rectifying the RF signal emitted by the RF scanner.
RF transceiver 104 transmits RF signals to RFID tag 110, and receives RF signals from RFID tag 110, via antenna 106. The data in transmit signal 108 and receive signal 112 can be contained in one or more bits for the purpose of providing identification and other information relevant to the particular RFID tag application. When RFID tag 110 passes within the operational range of RF interrogator 102, RFID tag 110 is excited and transmits data back to RF interrogator 102. A change in the impedance of RFID tag 110 can be used to signal the data to RF interrogator 102 via receive signal 112. The impedance change in RFID tag 110 can be caused by producing a short circuit across the tag's antenna connections (not shown) in bursts of very short duration. RF transceiver 104 senses the impedance change as a change in the level of reflected or backscattered energy arriving at antenna 106.
Digital electronics 114, which can comprise a microprocessor with memory, performs decoding and reading of receive signal 112. Similarly, digital electronics 114 performs the coding of transmit signal 108. Thus, RF interrogator 102 facilitates the reading or writing of data to RFID tags, e.g. RFID tag 110 that are within range of the RF field emitted by antenna 106. Finally, digital electronics 114 can be interfaced with an integral display and/or provide a parallel or serial communications interface to a host computer or industrial controller, e.g. host computer 116.
Also by way of background, Symmetric-key algorithms are a class of algorithms for cryptography that use trivially related, often identical, cryptographic keys for both decryption and encryption. The encryption key is trivially related to the decryption key, in that they may be identical or there is a simple transform to go between the two keys. The keys, in practice, represent a shared secret between two or more parties that can be used to maintain a private information link.
Symmetric-key algorithms can be divided into stream ciphers and block ciphers. Stream ciphers encrypt the bits of the message one at a time, and block ciphers take a number of bits and encrypt them as a single unit. Blocks of 64 bits have been commonly used; the Advanced Encryption Standard (AES) algorithm approved by NIST in December 2001 uses 128-bit blocks. Some examples of popular and well-respected symmetric algorithms include Twofish, Serpent, AES (aka Rijndael), Blowfish, CAST5, RC4, TDES, and IDEA.
Unlike symmetric algorithms, asymmetric key algorithms use a different key for encryption than for decryption. I.e., a user knowing the encryption key of an asymmetric algorithm can encrypt messages, but cannot derive the decryption key and cannot decrypt messages encrypted with that key. Symmetric-key algorithms are generally much less computationally intensive than asymmetric key algorithms. In practice, asymmetric key algorithms are typically hundreds to thousands times slower than symmetric key algorithms.
One disadvantage of symmetric-key algorithms is the requirement of a shared secret key, with one copy at each end. In order to ensure secure communications between everyone in a population of (n) people a total of n (n−1)/2 keys are needed, which is the total number of possible communication channels. To limit the impact of a potential discovery by a cryptographic adversary, they should be changed regularly and kept secure during distribution and in service. The process of selecting, distributing and storing keys is known as key management, and as noted is difficult to achieve reliably and securely. For example, symmetric ciphers have historically been susceptible to known-plaintext attacks, chosen plaintext attacks, differential cryptanalysis and linear cryptanalysis.
When used with asymmetric ciphers for key transfer, pseudorandom key generators are nearly always used to generate the symmetric cipher session keys. Lack of randomness in those generators or in their initialization vectors is disastrous and has led to cryptanalytic breaks in the past. Therefore, it is essential that an implementation uses a source of high entropy for its initialization.
In cryptography, an initialization vector (IV) is a block of bits that is required to allow a stream cipher or a block cipher to be executed in any of several streaming modes of operation to produce a unique stream independent from other streams produced by the same encryption key, without having to go through a (usually lengthy) re-keying process.
The size of the IV depends on the encryption algorithm and on the cryptographic protocol in use and is normally as large as the block size of the cipher or as large as the encryption key. The IV must be known to the recipient of the encrypted information to be able to decrypt it. This can be ensured in a number of ways: by transmitting the IV along with the packet, by agreeing on it beforehand during the key exchange or the handshake, by calculating it, usually incrementally), or by measuring such parameters as current time, used in hardware authentication tokens such as RSA SecurID, VASCO Digipass, etc., IDs such as sender's and/or recipient's address or ID, file ID, the packet, sector or cluster number, etc. A number of variables can be combined or hashed together, depending on the protocol. If the IV is chosen at random, the cryptographer must take into consideration the probability of collisions, and if an incremental IV is used as a nonce, the algorithm's resistance to related-IV attacks must also be considered.
IVs are implemented differently in block ciphers and in stream ciphers. In straight-forward operation of block ciphers or so-called Electronic Code Book (ECB) mode, encryption of the same plain text with the same key results in the same cipher text, this results in a considerable threat to security. Use of an initialization vector linearly added to (XORed with) the first block of plaintext or included in front of the plaintext prior to encryption in one of the streaming modes of operation solves this problem.
In stream ciphers, IVs are loaded into the keyed internal secret state of the cipher, after which a number of cipher rounds is executed prior to releasing the first bit of output. For performance reasons, designers of stream ciphers try to keep that number of rounds as small as possible, but because determining the minimal secure number of rounds for stream ciphers is not a trivial task, and considering other issues such as entropy loss, unique to each cipher construction, related-IVs and other IV-related attacks are a known security issue for stream ciphers, which makes IV loading in stream ciphers a serious concern and a subject of ongoing research.
The 802.11 encryption algorithm called WEP (short for Wired Equivalent Privacy), for example, used a “weak IV” that was led to it being easily cracked. Packet injection allowed for WEP to be cracked in times as short as 15 minutes or less. This ultimately led to the deprecation of WEP.